1. Technical Field of Present Example Embodiments
The present invention is concerned with the transmission of digitally coded video signals, for example over a telecommunications network, and, more particularly, video signals which have been encoded using a compression algorithm.
2. Description of Related Art
The rationale of compression algorithms is to exploit the inherent redundancy of the original video signal so as to reduce the number of bits that require to be transmitted. Many such algorithms are defined in international standards such as the ITU H.263 and the ISO MPEG standards. A useful review of these is given in Ghanbari, M., Video Coding, an introduction to standard codecs, IEE, London, 1999.
The degree of redundancy naturally varies with the picture content, and consequently the compression efficiency does too, resulting in a varying number of coded bits per frame. One option is to transmit the bits as they are generated, as in so-called variable bit rate (VBR) systems in which the transmitted bit rate varies considerably with time. Another option—constant bit rate (CBR) systems—is to employ a buffer at both the transmitter and receiver, to smooth out these fluctuations, and transmit from the transmit buffer to the receive buffer at a constant rate. CBR systems utilise a feedback mechanism to vary the rate at which data are generated (for example by adjusting the coarseness of quantisation used, or frame dropping), to prevent buffer overflow. The use of buffering necessarily involves the introduction of delay, increasing the latency of start (LOS)—i.e. the user has to wait while the receive buffer is filled to the necessary level before decoding and display of the pictures can commence. The feedback mechanism involves reduction in picture quality.
It has also been proposed to employ a degree of buffering to reduce, yet not totally eliminate, bit-rate variations (see, for example, Furini, M. and Towsley, D. F., “Real-Time Traffic transmissions over the Internet”, IEEE Transactions on Multimedia, Vol. 3, No. 1, March 2001).
A major consideration when transmitting over a telecommunications network, and in particular packet networks such as the internet, is the effect of network congestion, where packet loss and unpredictable delays can cause problems. This has given rise to proposals for reservation systems, where a transmitter can request the network to allocate a specified guaranteed bit rate for its transmissions for a period of time. One such system, called “RSVP” is described in the Internet Engineering Task Force (IETF) document RFC 2205. However, other systems such as Expedited Forwarding of Differentiated Service, or CR-LDP may also be used.
In the case of a live video feed, the future characteristics of the bitstream being coded are unknown; with recorded material, however, they are. The fact that reservation systems allow the amount of the reserved bit-rate to be changed offers the opportunity to decide on a policy of how much network capacity to reserve at any time, based on knowledge of the coded material. A simple approach is to calculate the peak (VBR, unbuffered) bit-rate and request this for the entire duration of the transmission, but this is wasteful of network capacity and of course the higher the capacity requested, the greater is the probability of the network being unable to provide it and hence of the reservation request being refused. Another simple approach, which minimises the bit-rate to be requested, is to calculate the average bit-rate of the whole transmission and request this; however this results in the need for a very large buffer at the receiver and, more importantly (given that large amounts of storage are today relatively cheap) a large LOS. A modification to the peak-rate approach is considered in the above-cited paper by Furini and Towsley. Their scheme involves identifying the point in the video sequence at which the peak rate reaches a maximum, and requesting this rate for the period of time up to that point. Then the maximum peak rate over the remainder of sequence is located, and this (lower) rate requested. This process continues in the same manner over the whole sequence. The paper also suggests that a degree of buffering might be applied, thereby reducing the effective peak rates before the reservation algorithm is applied. Although this system improves the efficiency of network use as compared with the single peak rate system, there is still much wasted (i.e. reserved but unused) network capacity, and of course the benefit is small if the maximum peak rate occurs towards the end of the sequence. It does however have the benefit that the amount of network capacity requested falls, and, specifically, a reservation request never asks for a bit-rate that exceeds that of the previous requests, thereby reducing the risk of the reservation request being refused.